Modern electronic products, such as those including microprocessors, often include digital systems and subsystems having binary output terminals which must be set to predetermined logic levels in response to the initial application of electrical power. This setting of the output levels is required before such systems and subsystems can begin operation. For instance, a dynamic RAM controller circuit includes a pair of interconnected flip-flops which have output terminals that must be set or reset to predetermined binary levels in response to the initial application of the power supply voltage. Also, multiplexers sometimes include counters having output terminals that must be set or forced to particular binary levels in response to the initiation of the power supply voltage. There are many other kinds of circuits which also require that output terminals thereof be set or forced to predetermined states in response to initial energization by a power supply. Oftentimes these circuits are provided in monolithic integrated circuit form.
In the past, the output levels of such systems and sub-systems were forced to desired known predetermined states by internal reset circuitry driven by signals derived from external sources. In the case of an integrated circuit system and subsystem, an external pin-out is required for coupling the internal reset circuitry to the external circuits, which generally provide a reset control potential of relatively low magnitude for a selected time in response to initial energization by the power supply. The internal reset circuitry responds to the reset control potential to cause the output signal states to assume the desired predetermined levels. The prior art circuits for providing the reset control voltage for the reset circuitry often include a reset switch or a discrete series circuit having a resistor connected in series with a capacitor, which provides a reset control voltage having a resistance-capacitance (RC) time constant at an output terminal therebetween. The reset circuitry is coupled to the output terminal.
The foregoing prior art solutions are particularly disadvantageous when applied to integrated circuits, which must be designed to fit into readily available packages which have a fixed number of pins associated therewith. To minimize the cost of the integrated circuit device, it is desirable to use packages with as few pins as possible. Thus, it is undesirable to require a pin for only the reset function. For example, such a requirement can make the difference between being able to utilize a standard 24 pin package instead of a standard 28 pin package. Use of the larger package not only costs more but takes up more space which can be particularly detrimental if a multiplicity of such packages are required, as may be the case with present computer boards. Moreover, the use of external switches, capacitors and resistors also increases costs, takes up space and complicates mechanical layout of the product. In addition, the manually resettable switch or RC time constant implementations for providing the reset signal sometimes cause an undesirable amount of time delay between the point in time when the product is initially energized and the point in time when the internal signal states have been set to the proper levels so that the product can begin operating. Furthermore, such implementations cannot be fabricated as part of an integrated circuit structure.